Genetic and biochemical studies with enterobacteria reveal that nitrogen control in these organisms is very complex. Nitrogen control can be defined as the enzymatic adjustment made by cells in response to the availability of nitrogen supplied as NH4 salts or amino acids in the growth medium. In general, when the nitrogen supply is limiting, the synthesis of the ammonia assimilatory enzymes, some amino acid transport systems, and some amino acid catabolic enzymes increasses. This aspect of nitrogen regulation is under the control of at least three regulatory loci. The function of the products of the genes g1nF, g1nG, g1nL has yet to be determined, although both positive and negative control mechanisms appear to be involved. The study of the positive aspects of nitrogen control under conditions saturating for negative control has been a principal focus of this laboratory. The results of these studies show that nitrogen control reflects the catabolic capacity of the cell and that utilizable nitrogen sources and some carbon sources are, to some extent, in competition for this capacity. Other studies have focused on the possibility that an additional regulatory role in nitrogen control might exist for some of the proteins which participate in the cascade for modulation of the enzyme activity of glutamine synthetase (GS). The evidence so far indicates and interaction between the elements of the activity modulation cascade and certain genetic elements involved in the regulation of synthesis. In order to clarify the relationship between the non-glnG/L, non-glnB type of constitutivity and adenyltransferase which was noted in some earlier work, genetic experiments to assign map position to mutations affecting these functions have continued.